Effect of potassium doping on electrical properties of carbon nanotube fibers

Abstract

We investigate the effect of potassium (K) doping on the transport properties of aligned single-walled carbon nanotube fibers. The temperature dependence of the electrical resistance, the current-voltage characteristics, and the magnetoresistance vs external magnetic field of the fibers consistently show that doping enhances the metallic character of the fibers and that the response of the samples can be quantitatively explained in two thermal regimes separated by a characteristic temperature T * . At temperatures higher than T * , the data are interpreted in the framework of variable range hopping theory, suggesting that the increased conductance with potassium doping is due to the increase of the density of states, which enhances carriers hopping. For temperatures below T * , experimental evidence of fluctuation temperature-induced tunneling mechanism suggests that the doping by K atoms affects the potential barriers established between adjacent carbon nanotubes, enhancing the metallic properties of the fibers. Carbon nanotubes (CNTs) are promising materials in electronics due to the potential wide range of applicability, 1 but one difficulty foreseen for large-scale developments is the fabrication of samples having homogeneous electrical properties. Aggregates, mats, or fibers are commonly formed by CNTs having both semiconducting and metallic properties, a peculiarity which limits the spectrum of possible applications. 2At present, several techniques are under investigation in order to overcome this problem, and one specific technique consists in doping the CNT aggregate in order to increase the charge carriers density and favoring a semiconducting-to-metallic transition of the whole aggregate.3 Metallic doping by using halogen 4 and alkali atoms 5 has been successfully investigated. In this Brief Report, we show that doping a single-walled CNT (SWCNT) fiber with metallic potassium atoms improves its electrical conductance, as reported for other types of CNT aggregates. 3,6,7 Moreover, interpreting the experimental data on the basis of the current theories for disordered noncrystalline materials, we provide a quantitative model for the role played by K atoms on the electrical properties of CNT fibers.The fibers studied here have an external diameter of 100 μm and consist of aligned CNTs having a diameter of 1 nm. 8In order to generate the K doping, discrete fiber pieces are loaded in a small vial (∼20 ml volume) and loaded uncapped into a bigger flask (150 ml) capped after the addition of ∼1 g of solid potassium. The enclosed system is baked up to 120• C to melt the potassium and saturate the flask with K vapors, and the fibers are kept in contact with the K vapors for 2 h. The presence of K inside the fibers is confirmed by energy dispersive spectroscopy (EDS), which detected a change of about 13% going from the surface to the center of the fiber, while the average value of the K concentration is 33%. An electron microscope image of the fiber surface is shown in the upper inset of Fig. 1 together with the s...

Understanding the Mechanical and Conductive Properties of Carbon Nanotube Fibers for Smart Electronics

“…So far, various studies have shown an excellent fit between Equation and the experimentally measured temperature dependence . The validity for CNT thin films was found in a much broad temperature range of 12–300 K .…”

“…The electron transport in CNT fibers, and more generally in CNT assemblies, is different from that in individual nanotubes. To understand the mechanism of electronic transport, there have been intensive investigations . According to the studies on the temperature, electric field, and magnetic field dependent electrical conductivity of CNT networks, 3D variable range hopping (VRH) is responsible for the electrical charge transport.…”

Transport mechanism in granular Ni deposited on carbon nanotubes fibers

“…Also shown is the normalized resistance of the bare fiber before the Ni electrodeposition which indicates its semiconducting character in the whole temperature range investigated. This effect has been previously studied 16 and a three-dimensional variable range hopping (VRH) regime 5 has been evidenced with a R vs T dependence well fitted by the expression R = R N e (T M /T ) 1/4 in the temperature range 3-50 K. Depositing Ni nanoparticles on the fiber surface generates an enhancement of their metallic properties. For short deposition times the fibers are still semiconducting with a negative slope in the R vs T curve.…”

Photoconductivity enhancement in alkali metal doped multiwall carbon nanotubes

“…On the other hand, many studies (for instance [13]) have shown that all the electrical and magnetic characteristics of the CNTs are changeable via doping procedure and theoretically it would be possible by doping alkali metals in the s-CNTs their semiconducting and also their photoconducting properties to be regulated for empirical applications (because doping of the alkali metals increase electron density in the conduction band of the doped CNTs). Salvato et al (2011) [14] have investigated the effect of potassium doping on the transport properties of aligned single-walled CNT fibers. They found that the I-V (current-voltage) characteristics and the magnetoresistance of the fibers consistently show that doping enhances the metallic character of the fibers and at higher than a characteristic temperature; enhancement in the K-doped sample is due to the increase of the density of states, which raises carrier's hopping.…”